1. Field of the Invention
The present invention relates to a galvannealed steel sheet used by press-forming for automobiles, home electrical appliances, building materials, and the like, and a producing method therefor, and, in particular, to a galvannealed steel sheet having an excellent sliding property (a flaking resistance), powdering resistance, chemical conversion treatability, and no uneven appearance, and a producing method therefor. This application is a national stage application of International Application No. PCT/JP2009/062538, filed on Jul. 9, 2009, which claims priority to Japanese Patent Application No. 2009-023603, filed on Feb. 4, 2009, and Japanese Patent Application No. 2009-022920, filed on Feb. 3, 2009, the contents of which are incorporated herein by reference.
2. Description of Related Art
A galvannealed steel sheet has excellent weldability and coatability in comparison with a galvanized steel sheet. Therefore, the galvannealed steel sheet is widely used in a wide range of fields as an automobile body as a principal use, home electrical appliances, building materials, and the like.
The galvannealed steel sheet is produced by heating treatment after hot dip galvanization of a steel sheet in order to form an Fe—Zn alloy layer on the surface of a steel sheet. By the heat treatment, alloying reaction is initiated through interdiffusion of Fe in a steel sheet and Zn in a galvanizing layer. It is said that the alloying reaction is preferably initiated from grain boundaries of a steel sheet. However, if many elements segregated easily in grain boundaries (grain boundary segregation elements) are contained in a steel sheet, interdiffusion of Fe and Zn is locally prevented. Therefore, an alloying reaction becomes heterogeneous, and thereby there is a difference in the thickness of a galvannealed layer formed. Since a linear defect appears by the difference in the thickness of a galvannealed layer, the quality of the steel sheet is poor due to an uneven appearance derived from a linear defect. In particular, there is a problem in that the unevenness easily appears in a steel sheet containing many grain boundary segregation elements such as P for the purpose of increasing the strength of a steel sheet in recent years. The problem attributes to constraint of interdiffusion of Fe and Zn in concentrated zones of P during alloying of a galvanizing layer by heterogeneous concentration of P in areas of surfaces and grain boundaries of a steel sheet in heating of a steel sheet. Therefore, the rate of an alloying reaction between Fe and Zn varies with location, and thereby there is a difference in the thickness of a galvannealed layer formed. The addition of inexpensive Si and/or Mn is widely used as a strengthening method for steel products. However, if the amount of Si in a steel sheet is more than 0.3 mass %, the wettability of a galvannealed layer is decreased significantly. Therefore, there is the problem in that quality of a galvannealed layer is poor and the quality of appearance is deteriorated.
For this reason, various galvannealed steel sheets having excellent quality of appearance has been investigated. For example, it is known that a method for producing a galvannealed steel sheet by dipped in a hot galvanizing bath after the surface of a steel sheet to be galvanized is ground so that an arithmetical mean deviation of profile (Ra) may be from 0.3 to 0.6 (for example, Patent Citation 1) and a method of forming a metallic coating layer such as Fe, Ni, Co, and Cu before hot dip galvanizing of an annealed steel sheet (for example, Patent Citation 2). However, in these methods, there is a problem in that since the extra process before hot dip galvanizing is required, the number of total processes increases and the cost increases with an increased number of facilities.
Typically, a galvannealed steel sheet is used after press-forming. However, a galvannealed steel sheet has a disadvantage of poor press formability compared with a cold-rolled steel.
The poor press formability results from a composition of a galvannealed layer. Typically, a Zn—Fe alloy layer formed by alloying reaction, which is diffused Fe in a steel sheet into Zn in a galvanizing layer, is a galvannealed coating layer (galvannealed layer) composed of Γ phase, δ1 phase, and ζ phase. In order of decreasing an Fe concentration, the galvanized coating layer is composed of Γ phase, δ1 phase, and ζ phase. In the order, the hardness and the melting point of each phase are decreased. Hard and brittle Γ phase is formed in an area of the galvannealed layer in contact with the surface of the steel sheet (an interface between the galvannealed layer and the steel sheet), and soft ζ phase is formed in an upper area of the galvannealed layer. ζ phase is soft and thereby adheres to press die easily, and has a high coefficient of friction and thereby has a bad sliding property. Therefore, when difficult press-forming is performed, ζ phase results in a phenomenon (flaking) in which a galvannealed layer adheres to a die and peels. Γ phase is hard and brittle, and thereby results in powdery peeling (powdering) of a galvannealed layer in press-forming.
A good sliding property is important in press-forming of a galvannealed steel sheet. Therefore, in view of the sliding property, an effective technique is that a galvanizing layer is alloyed to a high degree and thereby becomes a high Fe concentration layer having a high hardness, melting point, and adhesion resistance. However, powdering is caused by this technique in a galvannealed steel sheet produced thereby.
In view of powdering resistance, an effective technique is that a galvanizing layer is alloyed to a low degree and thereby has a low Fe concentration layer in which formation of Γ phase is suppressed which suppresses powdering. However, a galvannealed steel sheet produced by this technique has a poor sliding property and the poor sliding property results in flaking.
Therefore, both opposite properties of sliding property and powdering resistance are required so that a galvannealed steel sheet may have good press formability.
As a technique for improvement of press formability of a galvannealed steel sheet, a producing method (for example, the Patent Citation 3) for a galvannealed steel sheet having δ1 phase mainly is proposed. In the producing method, in a bath with a high Al concentration, galvanization is performed at a high temperature determined by the Al concentration, so that an alloying reaction may be suppressed, and then an alloying treatment, in which the temperature of a steel sheet is in the range of 460° C. to 530° C. at the exit of an alloying furnace which uses high-frequency induction heating, is executed. In addition, a producing method (for example, the Patent Citation 4) for a galvannealed steel sheet on which a galvannealed layer of single δ1 phase is formed is proposed. In the producing method, a hot dip galvanized steel sheet is held for 2 seconds to 120 seconds in a temperature area from 460° C. to 530° C. as soon as hot dip galvanizing of a steel sheet is performed, and then is cooled to 250° C. or less at a cooling rate of 5° C./s or more. Furthermore, a producing method (for example, the Patent Citation 5) for a galvannealed steel sheet, which determines a temperature pattern added up the values obtained by multiplying the heating temperature (T) by the heating time (t) at various times during heating and cooling of the steel sheet during the alloying treatment which results in a galvannealed steel sheet having both good sliding property and powdering resistance, is proposed.
The object of all conventional techniques is that by controlling the alloying degree, a galvannealed layer becomes hard and improves both powdering resistance and flaking resistance so as to reduce the disadvantages in press-forming of the galvannealed steel sheet.
Since sliding property is greatly influenced by a flat portion of surfaces, a producing method (for example, the Patent Citation 6) for a galvannealed steel sheet which has good powdering resistance and sliding property by controlling the flat portion in case of a galvannealed layer containing a large quantity of ζ phase in the surface layer is proposed.
The technique is a method for producing a galvannealed steel sheet which has a galvannealed layer containing a large quantity of ζ phase in the surface layer, good powdering resistance and sliding property by decreasing the alloying degree. However, it is considered that the galvannealed steel sheet is required to further improve flaking resistance (sliding property).
As a method for improving press formability of zinc alloy galvanized steel sheet, a method of applying a lubrication oil of high viscosity is widely used. However, there is a problem in that painting defects are formed in a painting process by insufficient removal of the lubrication oil since the lubrication oil has high viscosity, and a lack of oil in press-forming leads to unstable press performance. Therefore, a method (for example, the Patent Citation 7) of forming an oxide coat containing ZnO mainly on the surface of a zinc alloy galvanized steel sheet and a method (for example, the Patent Citation 8) of forming an oxide coat of Ni oxide is proposed. However, there is a problem in that the oxide films have bad chemical conversion treatability. Therefore, a method (for example, the Patent Citation 9) of forming an Mn based oxide film as a film for improvement of chemical conversion treatability is proposed. However, in all of the techniques of forming the oxide type film, the relationship between the oxide type films and a galvannealed layer has not been specifically investigated.
[Patent Citation 1] Japanese Unexamined Patent Application, First Publication No. 2004-169160
[Patent Citation 2] Japanese Unexamined Patent Application, First Publication No. H6-88187
[Patent Citation 3] Japanese Unexamined Patent Application, First Publication No. H9-165662
[Patent Citation 4] Japanese Unexamined Patent Application, First Publication No. 2007-131910
[Patent Citation 5] Japanese Unexamined Patent Application, First Publication No. 2005-54199
[Patent Citation 6] Japanese Unexamined Patent Application, First Publication No. 2005-48198
[Patent Citation 7] Japanese Unexamined Patent Application, First Publication No. S53-60332
[Patent Citation 8] Japanese Unexamined Patent Application, First Publication No. H3-191093
[Patent Citation 9] Japanese Unexamined Patent Application, First Publication No. H3-249182